As a vampire novel, I guess Scott Westerfeld's 'Peeps' would normally be classed as fantasy. However, Westerfeld has turned the idea on its head and given vampirism a proper scientific explanation - so I guess this book is actually science fiction. Hooray!!
Cal is a 19 year old virgin from Texas, who moves to New York and finds his life radically changed after a one night stand with a woman he meets in a bar (the product of far too many banana cocktails). Not only does he finds himself sans virginity, he's also gained something too - infection with an insidious parasite that makes him stronger, gives him excellent night vision and smell, and an almost debilitating libido. Fortunately he's only a carrier of the parasite and doesn't suffer from other side effects of the parasite - an anathema to everything you once loved, an affinity with rats, and a liking for human flesh. Unfortunately he has unknowningly infected several girlfriends, who are not so lucky and have become full blown parasite-positives (peeps for short). After being identified and recruited by the Nigh Watch (an ancient Buffy-like Watcher's Council that controls the vamp population) it's up to him to track his ex's down and bring them in for the cure. Cal still has questions about the woman who first infected him, however. Who was she and how can he track her down? Most importantly, why did she not just kill him?
Narrated by Cal, this book is fast paced and really captures the teen voice (something Westerfeld also did well in the Uglies Trilogy). Interspersed between the actual story chapters are alternate chapters detailing the life history all sorts of icky real-life parasites. By including these factual chapters, Westerfeld builds up all sorts of evidence supporting his theory. It's mind-blowing stuff and it feels like that the ordering and exposition in these factual chapters was just as important as the character and story development in the fiction. I did feel that Peeps had a better scientific grounding than the Uglies Trilogy, where lots is talked about recycling, energy efficiency, and *hoverboards* but most details are unsatisfactorily hazy.
In a world where even the author admits there are a no shortage of vampire novels, Westerfeld's take on vampyrism is both fresh and appealing, as are his characters. My only criticism is that Peeps feels like it's the first in a sequel, when apparently it's a stand alone novel. The whole novel is a slow build up, as Cal finds out more about the origins and history of the Peeps, and of the ancient evil that the Peeps must ready themselves to fight. However, the book is devoid of a real climax; Cal does fight the evil, but it is not vanquished. The novel ends with only a few of it threads tied satisfactorily and the characters (and this reader) waiting for the 'real show' to begin.
I would class this as an 'older' young adult book. The protagonists are all 19-plus and therefore only borderline teenagers. There are fairly frequent references to sex and some of the parasites are quite confronting too. I think it has great potential as an allegory (or mebbe it's more of a literal warning) on the dangers of sexually transmitted diseases, as Westerfeld manages to explore the consequences of STD transmission without any preaching. Bottom line, though, it's a just really fun read.
 Actually there is a sequel, 'The Last Days', but I'm not sure it's a sequel sequel, or more a novel set in the same universe. I'll let you know :-)